Conventionally, a thermal flowmeter is applied to an automotive engine for detecting a flow amount of intake air. The thermal flowmeter detects the flow amount in accordance with heat released from a hot wire of the flowmeter, for example.
When an engine is operated under a high load, pulsation increases in intake air due to reciprocation of a piston in the engine. Consequently, counterflow of intake air may occur under the influence of such intake pulsation. The thermal flowmeter cannot distinguish the flow direction between the counterflow and downflow. Accordingly, the thermal flowmeter may detect the amount of counterflow as the amount of the downflow. Consequently, as shown in FIG. 16, as the operation of the engine shifts to a high load condition on the right side in FIG. 16, counterflow is caused in the airflow, and the sensor output at normal condition indicated by the solid line in FIG. 16 becomes excessive as indicated by the dotted line in FIG. 16.
As shown in FIG. 17, JP-A-6-94497 proposes a flow detecting device that is provided with an obstacle 120 on the downstream side of an outlet of a bypass passage 110 formed in an air passage 100. The obstacle 120 is for avoiding excessive detection due to the counterflow, which is substantially opposite to the downflow indicated by the arrow in FIG. 17. In this airflow detecting device, the obstacle 120 is adapted to restricting the counterflow from entering into the bypass passage 110, so that the excessive detection can be reduced.
However, the obstacle 120 causes resistance in the airflow even in the downflow indicated by the arrow in FIG. 17. Consequently, flow velocity in the bypass passage 110 decreases, and detection of the flow amount becomes low in accuracy. In addition, the flow detecting device may become expensive due to additionally providing the obstacle 120.
Furthermore, the sensor in a thermal flowmeter has a heat capacity that causes delay in response. The amount of airflow and heat released from a heating element such as a hot wire has a relationship that presents an upwardly convex nonlinear characteristic. Accordingly, a sensor output tends to become small when pulsation increases, even when counterflow is not caused.
In recent years, intake components are integrated into a module. An integrated flowmeter is plugged in an outlet duct of an air cleaner. In this construction, it is difficult to additionally provide the obstacle 120 in the outlet duct. Accordingly, the integrated flowmeter needs to include a body having a structure for properly detecting airflow.